A seat rail device provided with an upper rail 1 engaged with and mounted onto a lower rail 2 such that an upper bottom wall section 11 is located above a lower bottom wall section 21 and also such that an upper sloped wall section 16 is located within a lower groove 26 surrounded by a lower wall section 23, a lower top plate section 25, and a lower downwardly extending section 27; a roller 31 provided and rollably supported between the lower bottom wall section 21 and the upper bottom wall section 11; an upper ball-shaped rolling body 33 provided and rollably supported in an upper in-groove space 26a partitioned and defined in the lower groove 26 by the upper sloped wall section 16; and a lower ball-shaped rolling body 35 provided and rollably supported in a lower in-groove space 26b.